An increasing demand for electronic equipment that is smaller, lighter, and more compact has resulted in a concomitant demand for semiconductor packages that have smaller outlines and mounting areas or “footprints.” One response to this demand has been the development of the ball grid array (BGA) semiconductor package, which “surface mounts” and electronically connects to a printed circuit board (PCB) with a plurality of solder balls. Another response has been the development of the “flip-chip” method of attachment and connection of semiconductor chips or “dice” to substrates (e.g., PCBs or lead-frames). Flip-chip mounting involves the formation of bumped contacts (e.g., solder balls) on the active surface of the die, then inverting or “flipping” the die upside down and reflowing the bumped contacts (i.e., heating the bumped contacts to the melting point) to fuse them to the corresponding pads on the substrate.
In both the BGA package and flip-chip mounting and connection methods, thermo-mechanical reliability is becoming an increasing concern of the electronic industry. Notably, the reliability of the solder joints is one of the most critical issues for successful application of such mounting and connection methods.
Within a flip-chip package, the integrated circuit die has solder bumps fused to corresponding pads on the substrate. These solder joints may be prone to cracks at high-stress points due to thermal stress cycling.
The Restriction of Hazardous Substances (RoHS) Directive limits the concentration of lead in components of electronic equipment. To comply with RoHS requirements, tin-lead solders are replaced with lead-free solders, such as tin-silver solders, that meet the RoHS requirement of a lead concentration of less than 0.1% by weight. However, this replacement adversely affects the thermo-mechanical reliability of solder joints because lead-free solders are generally more brittle than tin-lead solders.
One or more embodiments of the present invention may address one or more of the above issues.